This application relates generally to a method and apparatus for diffracting light, and more specifically to a diffraction grating useful in various applications, such as optical telecommunications, that require high diffraction efficiency in multiple polarization orientations.
The Internet and data communications are causing an explosion in the global demand for bandwidth. Fiber optic telecommunications systems are currently deploying a relatively new technology called dense wavelength division multiplexing (DWDM) to expand the capacity of new and existing optical fiber systems to help satisfy this demand. In DWDM, multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data. The carrying capacity of a fiber is multiplied by the number of DWDM channels used. Today, DWDM systems using up to 80 channels are available from multiple manufacturers, with more promised in the future.
Optical wavelength routing functions often use demultiplexing of a light stream into its many individual wavelengths, which are then optically directed along different paths. Subsequently, different wavelength signals may then be multiplexed into a common pathway. Within such routing devices, the optical signals are routed between the common and individual optical pathways by a combination of dispersion and focusing mechanisms. The focusing mechanism forms discrete images of the common pathway in each wavelength of the different optical signals and the dispersion mechanism relatively displaces the images along a focal line by amounts that vary with the signal wavelength.
Both phased arrays and reflective diffraction gratings may be used to perform the dispersing functions. While phased arrays are adequate when the number of channels carrying different wavelength signals is small, reflective diffraction gratings are generally preferable when large numbers of channels are used. However, reflective diffraction gratings tend to exhibit greater polarization sensitivity and since the polarization of optical signals often fluctuates in optical communication systems, this sensitivity may result in large variations in transmission efficiency. Loss of information is possible unless compensating amplification of the signals is used to maintain adequate signal-to-noise ratios. Although polarization sensitivity may generally be mitigated by increasing the grating pitch of the reflective grating, limitations on the desired wavelength dispersion for signals at optical telecommunication wavelengths preclude an increase in grating pitch sufficient to achieve high diffraction efficiency in all polarization directions.
It is thus desirable to provide a diffraction grating that can achieve high diffraction efficiency without significant polarization sensitivity when used at optical telecommunication wavelengths.
Embodiments of the present invention provide such a diffraction grating, achieving high diffraction efficiency in all polarization states when used for diffraction of an optical signal at telecommunications wavelengths. The diffraction grating in such embodiments includes a plurality of spaced triangular protrusions on a substrate in which reflective faces are blazed at angles xcex8b that are substantially different from the Littrow condition.
Thus, in one embodiment of the invention, the diffraction grating is configured to diffract an optical signal of wavelength xcex. It has a substrate and a plurality of reflective faces oriented at respective blaze angles xcex8b spaced along the substrate surface at a grating density 1/d. The blaze angles xcex8b substantially differ from the Littrow condition sin xcex8b=xcex/2d. Each of these reflective faces is supported by a support wall that is connected with the substrate surface such that the optical signal is reflected essentially only off the reflective faces and not off the support walls. Since the optical signal is reflected off the reflective faces but not the support walls, the diffraction efficiency of certain polarization states is improved.
In particular embodiments, the support walls are connected substantially normal with the surface of the substrate and in other embodiments they are connected at an obtuse angle with the substrate. The blaze angles are preferably within the range 50xc2x0xe2x89xa6xcex8bxe2x89xa670xc2x0 and more preferably within the range 50xc2x0xe2x89xa6xcex8bxe2x89xa660xc2x0. The density at which the reflective faces are spaced along the substrate is preferably between 700 and 1100 faces/mm and more preferably between 800 and 1000 faces/mm.
In a certain embodiment, the reflective faces are equally spaced along the surface of the substrate at density 1/d between 800 and 1000 faces/mm without exposing the surface of the substrate, with each of the blaze angles xcex8b substantially equal to 54.0xc2x0. In another embodiment, the reflective faces are equally spaced along the surface of the substrate at density 1/d between 800 and 1000 faces/mm such that a portion of the surface of the substrate is exposed between each such reflective face, with each of the blaze angles xcex8b substantially equal to 55.8xc2x0. In that embodiment, the support walls preferably have an altitude between 1200 and 1400 nm, more preferably substantially equal to 1310 nm.
In certain other embodiments, the diffraction grating is configured to be used to diffract the optical signal in an interference order higher than first order. In one such embodiment useful in second order for wavelengths between 1500 and 1600 nm, the reflective faces are spaced at a grating density of approximately 450 faces/mm, with blaze angles substantially equal to 55.8xc2x0. A portion of the substrate may be exposed between subsequent reflective faces, defining trenches between the faces. In one embodiment, each trench has a width between 0.50 and 0.70 xcexcm. The height of the support walls defines a groove depth, which in one embodiment is between 2300 and 2500 xcexcm.
Embodiments of the invention are also directed to a diffraction grating for diffracting an optical signal with a polarization-dependent loss less than 0.4 dB when the wavelength of the optical signal is between 1500 and 1600 nm. In one such embodiment, the grating is configured to diffract an optical signal having a wavelength between 1530 and 1565 nm with a polarization-dependent loss less than 0.04 dB.
According to embodiments of the invention, the diffraction grating is fabricated by forming two sets of parallel trenches in a crystal surface, one made with a crystalline-independent etching technique and the other made with a crystalline-dependent chemical etchant. The intersection of the two sets of trenches removes material from the crystal surface to produce an etched crystal surface that can be coated with a reflective material to form the diffraction grating or can be used as a master for batch fabrication of diffraction gratings.
In a particular embodiment, the first set of parallel trenches is initially formed perpendicularly from a surface of a silicon wafer. This set of trenches is then filled with a sacrificial material that also coats the surface of the wafer. The sacrificial material is subsequently patterned lithographically to expose the underlying wafer, with the crystalline-dependent chemical etchant being applied at the exposed portions. The deposited sacrificial material acts as an etch stop to the chemical etchant. Appropriate techniques for forming the first set of parallel trenches include reactive ion etching, deep reactive ion etching, and ion milling. Appropriate crystalline-dependent chemical etchants that preferentially stop etches along [111] orientations include KOH, hydrazine, and ethylene diamine pyrocatechol.
In another embodiment, plurality of parallel trenches are formed in a crystal surface with a crystalline-independent technique. Sacrificial material is deposited in each of the plurality of trenches, with some of the sacrificial material also being deposited on the crystal surface. The excess sacrificial material is removed from the crystal surface, such as by chemical and mechanical polishing (CMP). Subsequently, the crystal surface is exposed to a crystalline-dependent etchant. The resulting structure may be used for fabrication of the diffraction grating. Alternatively, the remaining sacrificial material may be removed from the structure before finalizing the grating fabrication.